During any manufacturing process it is important to perform testing at various stages of construction, especially at final test. This is true for the testing of electronic circuit components that operate at RF frequencies.
As production volumes of RF circuits increase world-wide, the need for fast, efficient, and accurate testing procedures becomes more important. Mainstream electronic devices, such as cell phones, are operating at ever-higher frequency bands. The RF components which comprise these devices now routinely operate in the 2 GHz range, and they are beginning to operate in the 5 GHz range and higher. The difficulty of maintaining high RF isolation among the ports of a circuit component under test becomes more difficult at ever-higher frequency bands.
RF circuit components are typically tested with instruments such as a multiport network analyzer. However, in order to use such a test instrument to measure the electrical characteristics of a circuit component, a system is needed that electrically connects the input/output (I/O) ports of the test instrument to the I/O ports of the component. Typically this is done as follows: coaxial cables, such as SMA cables, are screwed into a set of connectors (the I/O ports) on the test instrument. The other ends of the cables screw into a set of connectors which mate the coaxial cable to a printed circuit board. The printed circuit board carries the electrical signals (I/O's) to a test socket which provides an interface between the printed circuit board and the circuit component or device under test (DUT). It is crucial that this system containing coaxial cables, connectors, pc board, test socket, and the DUT be capable of providing an accurate measurement of the DUT. To get an accurate measurement of the DUT it is important that there be high RF isolation amongst both the signal paths (I/O's) and the signal return paths (ground paths) that travel between the network analyzer and the DUT. After the signals travel through the coaxial cables and onto the DUT, a good ground seal in which a continuous metal shield surrounds the signal pins, is hard to maintain, especially in a high-volume measurement system. As a result electromagnetic crosstalk or poor RF isolation between the signals and the signal return paths prevents an accurate measurement of the DUT.
Inductive or capacative coupling between the signal paths can, in principle, be calibrated out so that the DUT can be accurately measured. However, as is commonly the case in a test setup, a poor seal between the coaxial outer shield and the DUT's ground plane induces an effective inductance known as common ground inductance in the signal return path (ground) which can not be calibrated out of the system.
An example of a high-volume test socket used in measuring the S-parameters of an RF duplexer DUT is shown in FIGS. 5A, 6, 7 and 8. FIG. 5A shows test-set 50 with test bed surface 51 connected to coaxial cables 501–503. FIG. 5B shows test socket 70 connected via co-planar stripline pc board 513. The co-planar stripline pc board configuration is used so that the pc board signal trace is surrounded on the top, bottom, left and right side by grounded metal pc board traces, thus isolating the pc board signal traces from each other and to first order, forming a ground shield around the signal path. One side of each stripline signal trace is routed to the pc board connector (to the SMA cable) and the other side is routed to its respective test socket port.
FIG. 5C shows a typical test measurement system designed to achieve high RF isolation between the cables connecting measurement instrument 52 and device under test 53 as well as achieving near zero common ground inductance between the outer ground shield of the coaxial cables and the metal ground case of the “DUT”. Each signal (for example, signal 523a) should be electro-magnetically isolated from all other signals (for example, 523b, 523c). A coaxial cable, (such as coaxial cable 523a) with end connectors 521 which mate with connectors 520B, satisfies this condition in that a ground shield surrounds each signal trace 522a, 522b, 522c along the entire length from measurement instrument 52 to “DUT” 53. This test methodology is routinely applied in a low-volume manual test of a “DUT” in which the coaxial cables are screwed into the coaxial connectors on both the “DUT” and the measurement instrument.
In a production test environment, where the “DUT” is typically a planar part, it is difficult to achieve a proper seal between the outer coax and the ground plane of the “DUT”. This poor seal can reduce the isolation between the ports and can thus reduce the measurement accuracy (for example, a measure with an accuracy of ±1 dB) of the “DUT” from being obtained.
FIG. 6 shows existing test socket 60 having relatively poor (approximately 50 dB) port-to-port 602, 603, 604 isolation. Contacts 62, 63, 64 are used to contact the RF ports of the DUT with the stripline co-planar signal traces. A pair of “ground” pins 62a on either side of the contacts 62, 63, and 64 provide a current return (ground) path between the DUT and the stripline pc board ground metal. Because these pins only partially surround the signal pin there is substantial common ground inductance in the return ground path. Furthermore, RF electromagnetic radiation can stray from the port (for example, port 62) and couple to one of the neighboring ports (for example, ports 63, 64). In other words, this type of test socket does not provide a good ground shield around signal pins 62, 63, 64. Test socket 60 is shown for use with 11 mm×5 mm circuits.
FIG. 7 shows existing test socket 70 with center ground block 71 which provides better, but still insufficient, port-to-port isolation in the range of approximately 65 dB isolation. The center ground block provides a lower inductance signal path to the pc board co-planar stripline ground, and hence a lower inductance return path for each of the three signals, than does the test socket configuration of FIG. 6. Also, groundblock 71, placed between signal traces 72, 73, 74, helps to isolate the signal traces from one another. However, because the signal pins at each port is not fully surrounded by a metal shield, stray RF radiation can still couple from each port to its neighbors. In addition, the fact that radiation can escape or leak at the ports means that there is a common ground inductance between the DUT and the shield at each port.
FIG. 8 shows traditional high-volume test socket 80 designed to test 3.0 mm×3.0 mm filters (in ceramic packages). Signal pins 82 and 83 make contact to the input/output ports of the DUT and connect them to the co-planar stripline signal traces on the pc board. The other pins 85 and center ground block 81 provide a low inductance return path for the signal currents and hence provide low common ground inductance between DUT and shield and are connected to the pc board ground.
In order to guarantee +/−0.8 dB of measurement accuracy, port-to-port isolation that is approximately 20 dB higher than the dynamic range of the DUT is required. For circuits with a high dynamic range, e.g. 54 dB of dynamic range, such as an RF duplexer, at least 74 dB of isolation is required for an accurate (approximately 1 dB) RF transmission measurement.
For small parts, such as a filter in a 3.0 mm×3.0 mm ceramic leadless chip carrier (LCC) package as discussed with respect to FIG. 8, where 35 dB of dynamic range is required, at least 55 dB of port-to-port isolation is required for 1 dB of accuracy. In state-of-the-art test sockets, with imperfect metal shields surrounding the signal traces near the ports, and with the ports so closely spaced (˜3 mm), 55 dB of isolation is difficult to achieve. In this context the term “isolation” means that the electromagnetic cross-talk between any two test socket ports, in the desired frequency range, must be less than the specified value in all circumstances. For example, the isolation must be of the specified value when the test socket is empty, and when a metal short—that is a piece of metal that shorts all of the ports together—is placed inside the test socket.